An atomic force microscope (AFM) is a comparatively high-resolution type of scanning probe microscope. With demonstrated resolution of fractions of a nanometer, AFMs promise resolution more than 1000 times greater than the optical diffraction limit.
Many known AFMs include a microscale cantilever with a sharp tip (probe) at its end that is used to scan the specimen surface. The cantilever is typically silicon or silicon nitride with a tip radius of curvature on the order of nanometers. When the tip is brought into contact with a sample surface, forces between the tip and the sample lead to a deflection of the cantilever. One or more of a variety of forces are measured via the deflection of the cantilevered probe tip. These include mechanical forces and electrostatic and magnetostatic forces, to name only a few.
Typically, the deflection of the cantilevered probe tip is measured using a laser spot reflected from the top of the cantilever into a position detector. Other methods that are used include optical interferometry and piezoresistive AFM cantilever sensing.
In many AFMs, a feedback mechanism is employed to maintain the angular deflection of the tip nearly constant. The required movement of the tip to maintain the constant angular deflection provides a map of the area s=f(x,y) representative of the topography of the sample.
One component of AFM instruments is the actuator that maintains the angular deflection of the tip that scans the surface of the sample. Most AFM instruments use three orthonormal axes to scan the sample. The first two axes (e.g., X and Y axes) are driven to raster scan the surface area of the sample with typical ranges of 100 μm in each direction. The third axis (e.g., Z axis) drives the tip orthogonally to X and Y for tracking the topography of the surface.
Generally, the actuator for Z axis motion of the tip to maintain a near-constant deflection requires a comparatively smaller range of motion (e.g., approximately 1 μm (or less) to approximately 10 μm). However, as the requirement of scan speeds of AFMs increases, the actuator for Z axis motion must respond comparatively quickly to variations in the surface topography. For example, to scan at comparatively high speed (≧0.5 frames/sec or approximately 250 Hz or greater) and maintain suitable image quality requires a Z-axis actuator system capable of closed-loop response typically 20 kHz or higher. Known actuators capable of such speeds or ranges of motion are generally limited to piezoelectric technology. Unfortunately, known piezoelectric actuator designs with a range of motion as great as approximately 10 μm have a comparatively high capacitance and are difficult to drive at high frequency without incurring resonance-related drawbacks that limit closed-loop response. For this reason known Z axis actuators are configured to operate at much slower scan rates (typically 0.5 min/frame to 3 min/frame) to achieve good image quality, or must sacrifice scan range (typically less than 1 μm).
There is a need, therefore, for a piezoelectric actuator for an AFM that overcomes at least the shortcomings of known actuators discussed above.